As is well known in the art, the concept of handover refers to the process of changing the server or set of servers that communicate payload data with a user equipment. Typically, each server serves a different area of wireless coverage, and a cellular radio base station may be equipped with several servers. The terms handover and handoff are generally used interchangeably in the art.
The process of handover has evolved between generations of cellular wireless; first and second generation systems employed what may be termed hard handover, in which data payload communication to a user was transferred from a single base station to another base station.
In third generation systems, such as UMTS release 99 using code division multiple access (CDMA), so-called soft handover is used, and involves several servers within an active set simultaneously transmitting the same payload data to a user equipment. The user equipment then combines the payload data using a combining algorithm; this is a robust system, in which the redundancy of having two or more base stations serving a user equipment has the effect that communication may be maintained even when communication between the user equipment and another server has failed due to shadowing, multipath fading, interference or other problems occurring in the transmission path. Thus, soft handover provides improved quality of service over hard handover. However, the simultaneous transmissions make demands on radio resource that could otherwise be used to transmit payload data.
In third generation evolutionary systems and fourth generation systems, such as HSPA (‘High Speed Packet Access’), and LTE (Long Term Evolution), handover again relates to the selection of a set of servers with which signalling is maintained, corresponding to the active set of a CDMA system, but in addition there is a process of selection of the best server within the set for data payload transfer, potentially on a packet-by-packet basis, a process known as best server selection and also known as re-pointing or fast server selection. Signalling is maintained between each user equipment and the set of servers but payload data is only sent between the best server and the user equipment, thus making efficient use of radio resource. However, it is none the less necessary to send duplicate data across the backhaul network to each server in the set in order that the data is available for selection should a given server be selected as best server. The disadvantage of sending duplicate data is that this places demands on backhaul resource.
The server within the set that provides the highest pilot signal power received at a user equipment over a measurement period is known as the primary server. A pilot signal is a component of a signal that is transmitted at a known amplitude; in the case of a CDMA, a pilot signal is typically a signal component that is transmitted with a scrambling code but not a Walsh spreading code. In the case of an orthogonal frequency division multiplexed (OFDM) signal, a pilot signal may comprise one or more subcarriers that are transmitted with predetermined amplitudes and phases at predetermined times and frequencies. Measuring the power of a pilot signal is thus a reliable way of determining a measure of the signal power, since variations due to modulation with unpredictable payload data are removed. The power of a pilot signal is however not the only possible measure of received signal power; for example, an average of a received signal strength indicator (RSSI) may be used to indicated received signal power. It should be understood that when reference is made to the power of a received signal, that power may be measured in terms of the power of a pilot signal or by other methods known in the art.
As user equipments move within a network between areas of coverage of different servers, the set and indeed the primary server will change. The network continually determines which servers should form the set for a given user equipment based, for example, on the received power of base station pilot signals as measured by the user equipment and reported to the network.
For example, FIG. 1 illustrates a user equipment 1 in communication with a set of servers 2a and 2b but not in communication with a third server 2c. The servers 2a, 2b, 2c are connected to a radio network controller 3 via a telecommunications network 5. Typically the radio network controller controls the handover process. Thresholds are typically set by network operators to determine when to add or drop a server from the set for a user equipment in dependence on the measured received signal powers. Such thresholds are typically set in terms of received power of the server signal relative to the received power of the primary server signal. This may be expressed as a window of powers between the threshold power and the received power of the signal originating from the primary server, that is to say a power level range relative to the power of the signal associated with the primary server. A server may be added if its received power falls within the window, or above the threshold. The difference between the threshold and the received power of the primary server signal may be termed a margin. The margin is typically expressed in decibel (dB) terms; a difference in decibel values corresponds to a ratio of power levels expressed in linear terms.
FIG. 2 illustrates an example of a physical layout of servers 2a, 2b and 2c, their respective areas of coverage 4a, 4b and 4c, and the trajectory 6 of a user equipment moving relative to the servers.
FIG. 3 illustrates how the received power from the respective servers at a user equipment varies as the user equipment moves along the trajectory 6; received powers from servers 2a, 2b and 2c are indicated by curves 8a, 8b and 8b respectively. A threshold 10 is shown and the difference 12 between the threshold and the primary server power (the primary server power being shown by the sections of 8a and 8b shown as bold lines) is the margin, while the power range within the difference 12 is the window as previously mentioned. In the example of FIG. 3 a single threshold is used for determining the adding or dropping process. It can be seen that as the user equipment moves from point A via B to C that server 2b will be added to an active set at position 9, as the power 8b exceeds the threshold 10. At point 11, server 2c will also be added, and at point 13 server 2a, assumed already a member of the set, will be dropped, followed by the dropping of server 2b at position 15.
In practice, different add and drop thresholds may be selected (relative to the primary server power), so that hysteresis is provided meaning that servers are not repeatedly being added or dropped to the active set as they fall above or below a single threshold. In the case of CDMA systems, typical operator settings would have the effect of adding to an active set any server which has a pilot power as measured by the user equipment of 4 dB lower than the primary server pilot power or better, and to drop from the active set any server which has a pilot power as measured by the user equipment of 8 dB lower than the primary server pilot power or worse.
Practical systems differ from the simple situation illustrated by FIG. 3 in that there are time constants involved in the process of adding or dropping servers. Typically, when a received signal from a server exceeds a threshold, a timer is started and if the threshold is still exceeded when the timer times out, then the process of adding the server to a set, typically an active set, takes place. Similarly, when a received signal from a server falls below a threshold, a timer is started and if the signal still falls below the threshold when the timer times out, then the process of dropping the server from a set can take place. The thresholds and the times between the start and end of a count, that is to say when the timer counts out, need not be the same for adding as for dropping a server from a set. This time delay may be imposed to prevent the adding or dropping of servers due to transitory changes in signal powers.
In addition to the time taken to decide to add or drop a server, there is also a period of time required to implement the adding or dropping process. Taking the example of adding a server to an active set in a CDMA system, the process typically involves signalling from the user equipment to one or more servers and from there to a radio network controller to indicate that the threshold has been passed to the required certainty. The radio network controller will then typically make a decision as to whether or not to add a server to the active set on the basis, for example, of available resources. A message will then need to be sent to the existing members of the active set of servers serving the user equipment indicating to the user equipment that it should expect to receive signals from the server joining the active set, communicating amongst other data the Walsh code that new server will be using. This message is then required to be passed on from the members of the existing active set, or from a sub-set of them, that may include only one server, to the user equipment. If the signal received from all of the existing active set falls below a minimum level, then communication to the user equipment may not be possible. The generation, sending and receiving of the above messages takes time, and if the communication from the existing active set is lost before the user equipment receives the information that a new member of the active set has been added, then the handover process may fail and any call taking place may be dropped. The detail of the messaging may vary between systems, but typically there is a time delay between the crossing of a power threshold and communication being established between a user equipment and a server newly added to a set.
If a greater number of servers is maintained in the active set, for example by setting a lower threshold for adding a server to the active set, there may be a greater probability that communication may be maintained with at least one server, and so there may be a lower rate of dropped calls. However, as has been mentioned, this is at the expense of network capacity.
Typically, a network operator is able to balance the need for efficient utilization of network capacity (particularly on the downlink) and quality of service (for example probability of dropped calls) through network planning, including geographical cell planning, server selection, antenna orientation, and through choice of power thresholds or windows for adding and dropping servers from an active set. So it can be seen that whilst it is undesirable to have too many servers in the active set for a given user equipment, too few can result in quality of service issues.
One problem with the currently implemented methods described above is that they tend to be network-wide; that is to say that thresholds or windows for adding or dropping servers from active sets are specified across an entire network. These network-wide thresholds or windows do not take into account the differences in the radio environment that individual user equipments may be experiencing and are thus likely to be suboptimal for any given region of the network.